causes the fragmentation and swelling of lignocellulosic
biomass. The use of microwave helps to breakdown the
lignocellulosic material through the molecular collision
produced by dielectric polarization (Aguilar-Reynosa et al.
2017). The advantages of the microwave irradiation method
include (a) reduced reaction time, (b) enhanced digestibility,
(c) reduced energy consumption, (d) convenient operations,
and (e) minimal inhibitor formation in the process (Salakkam et al. 2019; Kumar and Sharma 2017). However, the
method suffers a few drawbacks, including an uneven radiation distribution in the heterogeneous mixture or bulk
materials that ultimately affects the process’s overall efficiency (Salakkam et al. 2019). The combination of chemical
reagents with microwave irradiation has shown a substantial
increase in the yield of sugar while decreasing the biomass’s
lignin content (Kumar and Sharma 2017). Hu and Wen have
reported the use of alkali reagents along with microwave
irradiation to yield 70–90% sugars from the switchgrass
(2008). Cheng et al. described the maximum reducing sugar
yield of 69.3 g/100 g TVS (total volatile solid) under
microwave heating of 50 g/L rice straw at 140 °C for
15 min in 0.5% sodium hydroxide (NaOH) solution (2011).
Ultrasonication is a relatively new technology and has
been used on a lab scale for lignocellulose pretreatment.
Ultrasound treatment forms small cavitation bubbles, which
is responsible for the breaking of hemicellulose and cellulose
fraction and hence increase the accessible area for further
hydrolysis (Patil et al. 2020). Various studies have shown
that the reactor type, its geometry, solvent, and biomass
characteristics, along with ultrasonic frequency, are critical
aspects of this pretreatment method (Kumar and Sharma
2017; Kumar et al. 2009). Montalbo-Lomboy et al. have
shown five to six times increase in the sugar yield from corn
starch slurry by using ultrasonication compared to the control method (2010). Although these modern techniques have
proved their potential in lignocellulose pretreatment, the
process’s overall cost is very high, making the process less
desirable. Moreover, such pretreatments require a piece of
corresponding equipment that adds up the total energy
consumption. Therefore, the use of these methods is relatively limited. However, a novel approach to integrate
chemical and physical processes can make these methods
economically feasible.
2.2 Chemical Pretreatments
Chemical methods include the use of various chemicals,
such as alkali, acid, organosolv, and ionic liquid for the
pretreatment. The acid and alkali pretreatment is the most
commonly used method that can be further employed with
ionic liquids concerning their significant chemical
properties.
Acid Treatment
The acid method generally employs acid in dilute or strong
concentration. The dilute acid pretreatment occurs at an
elevated temperature, while the strong acid requires mild
temperature conditions. Dilute acid pretreatment can obtain
lignocellulosic material with improved porosity and
enhanced enzymatic hydrolysis. Strong acid pretreatments
have a flexible choice of feedstock and can boost the yield of
monomeric sugar. However, both methods have a few
shortcomings, including concerns over corrosion of the
instruments, high cost, and requirement of the neutralization
reaction that forms salts (solid waste) (Harmsen et al. 2010).
Several acids, such as nitric acid, sulfuric acid, hydrochloric
acid, and carboxylic acids (maleic and oxalic acid), have
been used for the pretreatment of a variety of lignocellulose
(Kumar and Sharma 2017). Using diluted sulfuric acid at
121 °C for pretreatment of corn stover showed a substantial
increase in the breaking of hemicellulose and lignin from the
lignocellulosic structure (Cao et al. 2014). Zhang et al.
reported acid pretreatment of cornstalks with dilute
hydrochloric acid (0.2% HCL), which resulted in a 46-fold
increase in the hydrogen production compared to that of raw
corn stalk with a cumulative maximum H 2 yield of
149.69 mL/g of TVS (2007). Fan et al. reported a 136-fold
rise in the amount of hydrogen production from the wheat
straw waste when treated with acid (2% HCl) under microwave heating compared to raw wheat straw waste (2006).
The use of concentrated acid, i.e., strong acid, causes the
synthesis of highly concentrated inhibitors during the
degradation of cellulose. On the contrary, diluted acid helps
to hydrolyze both cellulose and hemicellulose while producing a minimum concentration of inhibitors (Kumar and
Sharma 2017).
Alkaline Treatment
Alkaline pretreatment methods are generally used to degrade
lignin present in biomass and usually performed at ambient
conditions. Typically, alkaline reagents break the ester and
glycoside bonds between lignin and hemicellulose, leading
to a change in the lignin structure, decrystallization of cellulose, and an increase in the solvation of hemicellulose
(Salakkam et al. 2019; Kumar and Sharma 2017). Among
various alkaline reagents (calcium hydroxide, potassium
hydroxide, sodium hydroxide, and ammonium salts), sodium
hydroxide was found to be the most active reagent for alkali
pretreatment of lignocellulose (Kumar and Sharma 2017).
Alkaline pretreatment can be carried out at moderate temperature and pressure. It requires non-corrosive and
non-polluting chemicals that help to make the process
environment friendly. Moreover, suppression of toxins and
inhibitors by alkaline pretreatment results in an enhanced
62
P. D. Patil et al.
biomass. The use of microwave helps to breakdown the
lignocellulosic material through the molecular collision
produced by dielectric polarization (Aguilar-Reynosa et al.
2017). The advantages of the microwave irradiation method
include (a) reduced reaction time, (b) enhanced digestibility,
(c) reduced energy consumption, (d) convenient operations,
and (e) minimal inhibitor formation in the process (Salakkam et al. 2019; Kumar and Sharma 2017). However, the
method suffers a few drawbacks, including an uneven radiation distribution in the heterogeneous mixture or bulk
materials that ultimately affects the process’s overall efficiency (Salakkam et al. 2019). The combination of chemical
reagents with microwave irradiation has shown a substantial
increase in the yield of sugar while decreasing the biomass’s
lignin content (Kumar and Sharma 2017). Hu and Wen have
reported the use of alkali reagents along with microwave
irradiation to yield 70–90% sugars from the switchgrass
(2008). Cheng et al. described the maximum reducing sugar
yield of 69.3 g/100 g TVS (total volatile solid) under
microwave heating of 50 g/L rice straw at 140 °C for
15 min in 0.5% sodium hydroxide (NaOH) solution (2011).
Ultrasonication is a relatively new technology and has
been used on a lab scale for lignocellulose pretreatment.
Ultrasound treatment forms small cavitation bubbles, which
is responsible for the breaking of hemicellulose and cellulose
fraction and hence increase the accessible area for further
hydrolysis (Patil et al. 2020). Various studies have shown
that the reactor type, its geometry, solvent, and biomass
characteristics, along with ultrasonic frequency, are critical
aspects of this pretreatment method (Kumar and Sharma
2017; Kumar et al. 2009). Montalbo-Lomboy et al. have
shown five to six times increase in the sugar yield from corn
starch slurry by using ultrasonication compared to the control method (2010). Although these modern techniques have
proved their potential in lignocellulose pretreatment, the
process’s overall cost is very high, making the process less
desirable. Moreover, such pretreatments require a piece of
corresponding equipment that adds up the total energy
consumption. Therefore, the use of these methods is relatively limited. However, a novel approach to integrate
chemical and physical processes can make these methods
economically feasible.
2.2 Chemical Pretreatments
Chemical methods include the use of various chemicals,
such as alkali, acid, organosolv, and ionic liquid for the
pretreatment. The acid and alkali pretreatment is the most
commonly used method that can be further employed with
ionic liquids concerning their significant chemical
properties.
Acid Treatment
The acid method generally employs acid in dilute or strong
concentration. The dilute acid pretreatment occurs at an
elevated temperature, while the strong acid requires mild
temperature conditions. Dilute acid pretreatment can obtain
lignocellulosic material with improved porosity and
enhanced enzymatic hydrolysis. Strong acid pretreatments
have a flexible choice of feedstock and can boost the yield of
monomeric sugar. However, both methods have a few
shortcomings, including concerns over corrosion of the
instruments, high cost, and requirement of the neutralization
reaction that forms salts (solid waste) (Harmsen et al. 2010).
Several acids, such as nitric acid, sulfuric acid, hydrochloric
acid, and carboxylic acids (maleic and oxalic acid), have
been used for the pretreatment of a variety of lignocellulose
(Kumar and Sharma 2017). Using diluted sulfuric acid at
121 °C for pretreatment of corn stover showed a substantial
increase in the breaking of hemicellulose and lignin from the
lignocellulosic structure (Cao et al. 2014). Zhang et al.
reported acid pretreatment of cornstalks with dilute
hydrochloric acid (0.2% HCL), which resulted in a 46-fold
increase in the hydrogen production compared to that of raw
corn stalk with a cumulative maximum H 2 yield of
149.69 mL/g of TVS (2007). Fan et al. reported a 136-fold
rise in the amount of hydrogen production from the wheat
straw waste when treated with acid (2% HCl) under microwave heating compared to raw wheat straw waste (2006).
The use of concentrated acid, i.e., strong acid, causes the
synthesis of highly concentrated inhibitors during the
degradation of cellulose. On the contrary, diluted acid helps
to hydrolyze both cellulose and hemicellulose while producing a minimum concentration of inhibitors (Kumar and
Sharma 2017).
Alkaline Treatment
Alkaline pretreatment methods are generally used to degrade
lignin present in biomass and usually performed at ambient
conditions. Typically, alkaline reagents break the ester and
glycoside bonds between lignin and hemicellulose, leading
to a change in the lignin structure, decrystallization of cellulose, and an increase in the solvation of hemicellulose
(Salakkam et al. 2019; Kumar and Sharma 2017). Among
various alkaline reagents (calcium hydroxide, potassium
hydroxide, sodium hydroxide, and ammonium salts), sodium
hydroxide was found to be the most active reagent for alkali
pretreatment of lignocellulose (Kumar and Sharma 2017).
Alkaline pretreatment can be carried out at moderate temperature and pressure. It requires non-corrosive and
non-polluting chemicals that help to make the process
environment friendly. Moreover, suppression of toxins and
inhibitors by alkaline pretreatment results in an enhanced
62
P. D. Patil et al.